1. Technical Field
The present invention relates to a method and a system for prediction and treatment of hydrodynamic and terrain-induced slugs being transported in a multi-phase flow line.
The method and the system according to the present invention can be adapted to any production system, e.g. flow line system or wellbore tubing, transporting a multiphase fluid towards a downstream process including a separator (two- or three-phase) or a slug catcher at the inlet, in which there is regulation of both pressure and liquid level(s). The multiphase fluid normally consists of a mixture of an oil (or a condensate) phase, gas and water.
2. Description of Related Art
A typical production system where the present invention could be implemented includes multiphase transport from platform wells, from subsea wells towards a subsea separator, from a subsea production template towards an offshore platform including a riser, between offshore platforms, from a subsea production system towards an onshore process facility or between onshore process facilities.
Depending on fluid properties, flow line characteristics and superficial velocities of the different fluid phases, a multiphase production system might give what is known as slug flow, experienced as fluctuating mass flow and pressure at the production system outlet. Further, if these slugs are “large” compared to the design of the downstream equipment, the fluctuations could propagate into the process and reach a level untenable to the operators. As a consequence, and as a precaution to avoid a process trip, there are numerous examples where multiphase production lines have been choked down due to incoming slugs.
Slugs are normally initiated in two ways that are fundamentally different. Terrain-induced slugs are caused by gravity effects when the velocity differences, and thus the interfacial friction, between the separate fluid phases is too small to allow the lightest fluid(s) to counteract the effect of gravity on the heavier fluid(s) in upward inclinations. Hydrodynamic slugs (identified in a flow regime envelope as a function of the pipe angle and the superficial fluid velocities for a given fluid) are formed by waves growing on the liquid surface to a height sufficient to completely fill the pipe. Because of differences in the velocities of the various fluid phases up- and downstream of this hydrodynamic slug, an accumulation of liquid and thus a dynamic slug growth can occur.
Hydrodynamic slugs too are affected by the flow line elevation profile, since their formation and growth depend on the pipe angles. Note, however, that an obvious way to prove the distinction between terrain-induced and hydrodynamic slugs is that hydrodynamic slugs could be formed in 100% horizontal flow lines (sometimes even in downwards inclination), whereas terrain-induced slugs somehow need an up-wards inclination.
Slugging is by definition a transient phenomenon, and steady state conditions are hard to achieve in a slugging flow line system. In such a system, hydrocarbon liquid (alternatively water or a hydrocarbon/water mixture) accumulates along the production system and the slugs will at some point reach the flow line exit. Between these slugs, there will be periods where small amounts of liquid exiting the system and the process will more or less receive a single gas phase, also described as gas slugs.
In order to overcome process disturbances due to slugging (terrain-induced or hydrodynamic), three methods have traditionally been used in multiphase transportation systems:                Reduce the flow rate and thereby the slug volumes within the limits of the downstream process, by throttling the inlet choke or by selecting a smaller flow line diameter in the design phase        Prolong start-up time or ramp up time when changing flow rates        Increase if possible the dimensions of the downstream process (i.e. slug catcher, alternatively the 1st stage separator)        
These “traditional” methods will either reduce production from the flow line systems in question or increase the costs and dimensions of the downstream process. Additionally, even if accounted for, slugs might grow larger than expected or could occur at unfortunate moments compared to actual process capabilities. As a consequence, the pressure and flow fluctuations could result in process shut-downs, which might have significant financial impacts.
Since every gas and oil producer wants to optimize the operating conditions of their process plants, there have been several attempts to find improved solutions to overcome process perturbations caused by slugging in the upstream production system.
U.S. Pat. No. 5,544,672 describes a system for mitigation of slug flow. It detects incoming slugs upstream of the separator and performs a rough calculation of their respective volumes. These slug volumes are thereafter compared with the liquid handling capacity of the separator. If the estimated volume of the incoming slugs exceeds the liquid slug handling capacity of the separator, a throttling valve located upstream of the separator is choked.
This solution has the advantage that it is simple and could be used for both hydrodynamic as well as terrain-induced slugs, since it is located downstream of the point where slugs are generated. However, the system entails some major disadvantages:                Since the flow rate is being throttled down, it has a negative impact on the production and thereby the field economics.        It does not take into account the slug handling capacity in the downstream process.        It does not describe how gas slugs are identified and treated. As a consequence pressure fluctuations in the separator due to incoming gas slugs must still be solved by gas flaring.        The system does not separate water slugs from hydrocarbon (HC) liquid slugs which could give process perturbations downstream of a three-phase separator.        It prolongs the start-up time after system shut-down, since the production is being throttled down every time a liquid slug is present.        
International Patent Application WO 01/34940 describes a small (mini-) separator located at the top of the riser just upstream of the 1st stage separator. Slugs are either suppressed by volumetric flow controller or liquid flow controller mode, depending on the slug characteristics. Regulation is achieved by two fast acting valves on the gas and liquid outlet streams downstream of the mini-separator, based on pressure and liquid level data from the mini-separator as well as flow rate measurements of its outlet streams.
Moreover, the International Patent Application WO 02/46577 describes a model-based feedback control system for stabilization of slug flow in multiphase flow lines and risers. The system consists of a single fast acting valve located at the outlet of the transport system, i.e. upstream of the separator. The opening of this valve is adjusted by a single output control signal from the feedback controller that uses continuous monitoring of pressure upstream of the point where slugs are generated as the main input parameter. This control system is specially suited for terrain-induced slugs since any liquid accumulation is detected by pressure increase upstream of the slug (due to static pressure across the liquid column). However, the system does not show the same performance for slugs which are hydrodynamic by nature since these slugs could be formed in perfectly horizontal flow lines, and thereby not cause a build-up of pressure upstream of the slug.
Briefly, for the two latter slug control systems, fast acting equipment located at the outlet of the transportation system, in combination with quick response time of the control loops are used to suppress development of slugs, by immediately counteracting the forces contributing to slug growth.
However, these solutions also entail several disadvantages:                As for the slug mitigation system they do not take into account the slug handling capacity in the downstream process.        The control system described in WO 02/46577 does not cater to hydro-dynamic slugs, while the system described in WO 01/34940 handles slugs which are terrain-induced by nature far better than hydrodynamic slugs.        They are normally not self-regulating for any operational range in the transport system, and the systems require manual input from an operator or must be de-activated during some of the normal production scenarios.        They both require fast acting valve(s) in combination with quick response time of the control loops.        They generalize on flow line systems including vertical piping (i.e. risers or tubing) at the outlet of the transport system.        The system described in WO 01/34940 requires topside equipment and could be costly, especially in the case of weight being an issue.        
Generally speaking, none of the existing systems fully integrates the transport system and the downstream process. Hence, they do not cover the full range of incoming slugs including hydrodynamic slugs as well as gas and water slugs. Finally, their application is limited to a narrow operating range and they require manual input or de-activation at some time.